The present invention relates to compositions and a process leading to colorless or light colored aqueous phenol-formaldehyde resoles.
Lignocellulosic composites primarily find use in construction or fabrication. These composites may be used in building construction or any fabrication where wood is a traditional material used. Absorption of water by the composites encourages growth of wood-rotting fungi and other microorganisms that detract from the appearance and functionality of the composites. Additional problems associated with water absorption are raised joints, buckling of panels, and unevenness. Furthermore, the poor dimensional stability of state-of-the-art lignocellulosic composites affects their mechanical properties and reduces their load carrying ability. Another result of poor dimensional stability is unevenness of roof and floor underlayments, and of building siding.
Lignocellulosic composite panels are conventionally manufactured by hot pressing lignocellulosic materials with wax and thermosetting resin. This is referred to as a conventional bonding process. The wax is a sizing agent to improve the water resistance of the once-formed composite. The resin is a binding agent that holds the materials comprising the composite together, thus forming them into a unitary shape. In the production of such panels, a phenolic binder, e.g. phenol-formaldehyde resin, is applied to cellulose components such as chips or veneers to form a mat or to otherwise consolidate the components. The consolidated components, also referred to as a mat herein, are then placed between two heated platens and pressed into the composite panels.
Conventional phenol-formaldehyde resoles that have low free phenol content and are clear or lightly colored typically have a low molecular weight. They are low in molecular weight because they are low in alkali content. Raising the alkali content results in a more highly colored resin. These low molecular weight resoles require more press time during the formation of the composite panels than would a high molecular weight resole. A high molecular weight resole would lessen the press time which would increase the efficiency of the production process of the panels.
The prior art phenol-formaldehyde resin used in the manufacture of lignocellulosic composites may be in the form of a solid or a liquid. Powdered phenolic resins, such as novolac, resole, or combinations thereof, may generally be used. U.S. Pat. No. 4,098,770 to Berchem, et al., discloses a typical spray-dried phenol-formaldehyde resin, modified with added non-phenolic polyhydroxy compounds, used in the manufacture of waferboard. Liquid phenol-formaldehyde resins, such as resole or resole and novolac combinations, may also be generally used in the manufacture of lignocellulosic composites. Parameters for the manufacture of either liquid or solid phenol-formaldehyde resins are disclosed in Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and L. A. Pilato, Springer-Verlag (1985)) and Advance Wood Adhesives Technology, (A Pizzi, Marcel Dekker (1994)).
Resoles are the result of the condensation of phenolic compounds with aldehydes (typically formaldehyde) in a strongly alkaline medium. After completion of the condensation reaction, the reaction mixture is neutralized with an acid, such as dilute HCl. Water mid excess phenol are then largely distilled off. In order to obtain a low phenol containing resole, a very strong alkaline solution is needed. Neutralization with a strong acid results in precipitation of salts.
U.S. Pat. No. 5,345,001 issued to Schroter et al.teaches a low free phenol, phenol-formaldehyde resole wherein the condensate is neutralized with an aromatic hydroxy carboxylic acid. Schroter et al. do not teach adding a metal chelating agent to the condensation mixture. Furthermore, the pH of the resins produced by Schroter et al are basic, and Schroter et al. do not teach acidifying the resins.
In the production of lignocellulosic composite panels, there is a need for a phenol-formaldehyde resole that has low amounts of free phenol, is clear or light colored, is water soluble, has a high molecular weight, stable in solution during storage, soluble at an acidic pH, and has low viscosity. Also the resole should produce a fast cure, and result in low water absorption and thickness swell of the panels.
Unlike previous technologies, embodiments of the present invention lead to clear, storage stable, fast curing, high molecular weight resins. These compositions are also capable of producing composite wood panels with superior water absorption and thickness swell test results when soaked in water.
The present invention is directed to an aqueous, high molecular weight, resole produced by a condensation of a phenolic compound with an aldehyde in the presence of a metal chelating agent, a strong base and a phenolic carboxylate, followed by acidification of the solution to a desired pH with an acid.
The present invention is also directed to a process for the preparation of an aqueous, high molecular weight, resole solution which includes: (a) heating a phenolic compound, an aldehyde, a strong base, a phenolic carboxylate, and a metal chelating agent at a temperature ranging from about 60xc2x0 to about 75xc2x0 C., forming a methylol solution, (b) condensing the methylol solution of step (a) at a pH ranging from about 8 to about 10, forming a polymer solution, and (c) acidifying the polymer solution of step (b) with an acid.
The present invention is further directed to a process for the preparation of an aqueous, high molecular weight resole solution which includes: a) heating a phenolic compound, a strong base, a phenolic carboxylate, and a metal chelating agent at a temperature ranging from about 60xc2x0 to about 75xc2x0 C., forming a solution; b) adding an excess of aldehyde to the solution of step (a) to form a mixture and condensing the mixture at about 90xc2x0 C. under a vacuum of about 11.8 inches of Mercury to a Gardner-Holdt viscosity of U to Z; c) cooling the mixture of step (b) to a temperature of about 70xc2x0 C., adding additional phenol to a desired molar ratio, and adjusting the mixture to a pH of about 9 with an alkali metal hydroxide while holding the mixture at about 70xc2x0 C. until the free formaldehyde falls to a desired level; and d) acidifying the solution of step(c) with an acid.
Another embodiment of the present invention is directed to a bonding resin that cures clear when employed in the production of composite wood panels containing the aqueous, high molecular weight resole solution of the invention.
The invention is further directed to an aqueous, high molecular weight, resole solution containing an acid and a condensate of a phenolic compound and an aldehyde, the condensate formed in the presence of a metal chelating agent, a strong base and a phenolic carboxylate.
A further embodiment of the invention is directed to a clear, cured resin comprising the aqueous, high molecular weight, resole solution of the present invention.